Space-based weaponry is the fundamental issue dividing the United States and the Soviet Union one month before the Geneva summit, according to Soviet officials. ``This issue is something relatively new. It has not been a main concern for very long, but it certainly is now,'' says one Soviet official.

With the US engaged in so much research on space weaponry, says another Soviet official, the Soviet Union simply cannot afford to ignore it.

Some Western diplomats hold out the hope that, with Soviet attention focused on the issue, the US will eventually be able to convince the Soviets that a space-based strategic-missile defense system can play a deterrent role.

The Soviets don't see it that way. In their view, little good can come out of a space-based defense system. They offer a number of explanations for that reasoning.

The bluntest one, and one that has notably faded since Soviet leader Mikhail Gorbachev took office in March, is that the US is secretly planning to construct a ``shield'' from behind which it can intimidate -- or, worse, dictate to -- the Soviet Union.

A more considered view is that the Soviet Union will suffer most if space-based weapons are deployed -- and that overall nuclear stability will not be enhanced.

Some Soviet officials argue that if the US does press ahead with plans to deploy weapons in space, the Soviet Union will have no choice but to respond. It will either be forced to step up its own research on space-based weapons systems, or build up its own nuclear arsenals to overcome any US defenses.

The Soviet Union is notably lagging behind the US in the kind of high-technology research that space-based weapons system require. Thus, it would start development of the system at a disadvantage -- a position no Soviet leader can find to his liking.

The other option -- building up the Soviet offensive arsenal -- would be exceptionally costly, and would siphon money away from other sectors of the Soviet economy that are falling further behind the West. Mr. Gorbachev conceded this linkage in an interview published last week in Time magazine.

For these reasons, the Soviets have as their top priority the halting of US research on the Strategic Defense Initiative (SDI, popularly known as ``star wars'').

In the interview with Time, Gorbachev suggested that the Soviets might countenance laboratory research on space-based weapons systems. That was an apparent attempt to address American concerns that a ban on space weapons research couldn't be verified. But Gorbachev would ban any actual construction of prototypes or models for space-based weapons.

The Soviet desire to defuse the SDI project is keen. Even the recent five-month Soviet moratorium on nuclear testing was announced with SDI in mind: Soviet officials know that if the US should agree to a moratorium, it would not be able to conduct some of the tests needed for SDI.

But the failure to go along would, of course, give the Kremlin an opportunity to play up its own peaceful intentions while at the same time castigating the US for failing to follow its lead.

Thus, the Soviets saw a moratorium as helping their cause either way. And some Western analysts concede it has done precisely that, especially in some Western Eu-ropean countries.

The courting of European public opinion continues even now, with Gorbachev meeting this week with Johannes Rau, a leader in West German's Social Democratic Party. It is an event sure to garner extensive publicity in Western Europe.

Gorbachev's meeting last week with US senators delighted Soviet officials. The senators left here speaking of ``new flexibility'' on the Soviet side that presaged ``radical'' arms reduction proposals from Gorbachev.

Indeed, Western Kremlin-watchers say that Gorbachev is cultivating an image of reasonableness, energy, and flexibility. And they add that his interviews, meetings with Western politicians, and carefully-orchestrated trips and speeches around the Soviet Union are planned with news-media coverage in mind.

Soviet officials, however, insist that they are not merely engaging in propaganda tactics. They say they genuinely want to see progress in curbing the arms buildup at the November Geneva summit between Gorbachev and President Reagan.

Going into the summit, many Western analysts here say they have not detected any substantive change in Soviet foreign policy since Gorbachev took office some six months ago.

For the most part, they say, he is hewing to the same positions espoused by Konstantin Chernenko and Yuri Andropov, and even Leonid Brezhnev before them.

``It's still a difference in style, not in substance,'' says one Western diplomat.